Rollup merge of #109621 - scottmcm:update-variantidx, r=compiler-errors

Refactor: `VariantIdx::from_u32(0)` -> `FIRST_VARIANT`

Since structs are always `VariantIdx(0)`, there's a bunch of files where the only reason they had `VariantIdx` or `vec::Idx` imported at all was to get the first variant.

So this uses a constant for that, and adds some doc-comments to `VariantIdx` while I'm there, since [it doesn't have any today](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_target/abi/struct.VariantIdx.html).

1 files changed, 2 insertions, 2 deletions

diff --git a/compiler/rustc_const_eval/src/interpret/place.rs b/compiler/rustc_const_eval/src/interpret/place.rs
index ff6db143ddf..a95bcaa3f99 100644
--- a/compiler/rustc_const_eval/src/interpret/place.rs
+++ b/compiler/rustc_const_eval/src/interpret/place.rs
@@ -8,7 +8,7 @@ use rustc_ast::Mutability;
 use rustc_middle::mir;
 use rustc_middle::ty;
 use rustc_middle::ty::layout::{LayoutOf, TyAndLayout};
-use rustc_target::abi::{self, Abi, Align, HasDataLayout, Size, VariantIdx};
+use rustc_target::abi::{self, Abi, Align, HasDataLayout, Size, FIRST_VARIANT};
 
 use super::{
     alloc_range, mir_assign_valid_types, AllocId, AllocRef, AllocRefMut, CheckInAllocMsg,
@@ -796,7 +796,7 @@ where
                 let variant_dest = self.place_downcast(&dest, variant_index)?;
                 (variant_index, variant_dest, active_field_index)
             }
-            _ => (VariantIdx::from_u32(0), dest.clone(), None),
+            _ => (FIRST_VARIANT, dest.clone(), None),
         };
         if active_field_index.is_some() {
             assert_eq!(operands.len(), 1);